This is a randomized, double-blind, placebo-controlled, multicenter, phase II study of Amgen's calcimimetic agent, AMG 073, in patients with primary hyperparathyroidism. Its main purposes are to: (1) determine the optimal dosage of AMG 073 that will return serum calcium and parathyroid hormone toward, or preferably to normal without undue side effects; and (2) determine the pharmacokinetics of AMG 073.